The effect of porogen on physical properties in MTMS–BTMSE spin-on organosilicates

Decreasing the circuit dimensions is driving the need for low-k materials with a lower dielectric constant to reduce RC delay, crosstalk, and power consumption. In case of spin-on organosilicate low-k films, the incorporation of a porogen is regarded as the only foreseeable route to decrease dielectric constant of 2.2 or below by changing a packing density. In this study, methyltrimethoxysilane (MTMS)–bis(trimethoxysilyl)ethane (BTMSE) copolymers that had superior mechanical properties than MSSQ (methyl-silsesquioxane) were blended with amphiphilic block copolymers used as sacrificial pore generators. While adding up to 40 wt.% porogen into MTMS:BTMSE = 100:50 matrix, optical, electrical, and mechanical properties were measured and the pore structure was also characterized by positron annihilation lifetime spectroscopy (PALS). The result confirmed that there existed a tradeoff in attaining the low dielectric constant and desirable mechanical strength, and no more pores than necessary to achieve the dielectric objective should be incorporated. When the dielectric constant was fixed to approximately 2.3 by controlling BTMSE and porogen contents simultaneously, the thermo-mechanical properties of the porous films were also investigated for the comparison purpose. Under the same dielectric constant, the concurrent increase in BTMSE and porogen contents led to improvement in modulus measured by the nanoindentation technique but deterioration of adhesion strength obtained by the modified edge lift-off test.     

Styrene butadiene rubber/aluminum powder composites—mechanical,morphological and electrical behaviors

Different weight fractions of aluminum (Al) powder viz., 10, 20, 30, 40, 50, 60 and 70 phr were incorporated into styrene butadiene rubber (SBR) matrix. The Al powder filled and vulcanized SBR composites have been characterized for mechanical properties such as tensile strength, tensile modulus and surface hardness. A drastical improvement in tensile strength and tensile modulus with increase in filler content of the composites was noticed. The electrical properties such as dielectric constant, tan delta and dielectric loss were measured for all the four compositions. The effect of volume fraction (0–70 phr) of conducting filler, frequency (100 kHz–30 MHz), temperature (25–75°C) and relative humidity on dielectric constant, dielectric loss and tan delta values of the composites were studied.     

High temperature flexural strength and fracture toughness of AlN with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic

The effects of temperature on the fast fracture behavior of aluminum nitride with 5 wt% Y₂O₃ ceramic were investigated. Four-point flexural strength and fracture toughness were measured in air at several temperatures (30–1,300 °C). The flexural strength gradually decreased with the increase of temperature up to 1,000 °C due to the change in the fracture mode from transgranular to intergranular, and then became almost constant up to 1,300 °C. Two main flaw types as fracture origin were identified: small surface flaw and large pores. The volume fraction of the large pores was only 0.01%; however, they limited the strength on about 50% of the specimens. The fracture toughness decreased slightly up to 800 °C controlled by the elastic modulus change, and then decreased significantly at 1,000 °C due to the decrease in the grain-boundary toughness. Above 1,000 °C, the fracture toughness increased significantly, and at 1,300 °C, its value was close to that measured at room temperature.     

Effects of hydrothermal aging on glass–fiber/polyetherimide (PEI) composites

The moisture absorption behavior and the influence of moisture on thermal and mechanical properties of glass–fiber/polyetherimide (PEI) laminates have been investigated. The laminates were exposed to hydrothermal aging at two different temperatures and high moisture rates. The properties of as-received and hydrothermally aged samples were compared. The hydrothermally aged laminates contained a large amount of moisture which caused decrease in the glass transition temperature and deterioration in mechanical properties (interlaminar shear strength, flexural modulus, bearing strength, etc.). Fractographic analysis revealed interfacial debonding as the dominant failure mechanism, indicating a strong influence of water degradation on fracture toughness results. Alterations in visco-elastic properties of glass/PEI composite which was exposed to hydrothermal aging were analyzed with the dynamic mechanical thermal analysis (DMTA) method. DMTA tests give evidence of plasticization of the PEI matrix.     

The assessment of the fracture behavior in spin-on organosilicates by nanoindentation and nanoscratch tests

Fracture behavior of the copolymers proposed as interlayer dielectric candidates is examined to find the adequate amount of bis(trimethoxysilyl)ethane to methyltrimethoxysilane that can optimize mechanical properties. In spite of the shortcoming that provides a result for comparison purpose, the indentation test becomes a good alternative to estimate cohesive and adhesive fracture strength simultaneously. The scratch test can also be used to evaluate adhesive fracture strength. The similarity between the scratch test and the chemical mechanical planarization process may offer a clue for predicting the possibility of failure during fabrication procedure. The sequence of magnitude in critical loads measured from both tests corresponds with that of fracture toughness assessed by the modified edge lift-off test.     

Nondestructive method of evaluating the shock resistance of composite (polymer-polymer) materials

A study is made of the connection between the fracture toughness of shock-resistant polymeric composites based on polycarbonate, polybutyleneterephthalate, and an ABS-copolymer and parameters characterizing mechanical and electrical relaxation in the region of local molecular mobility. A method is proposed for determining shock characteristicsa priori on the basis of the temperature-frequency dependences of the elastic modulus and mechanical loss modulus. The difference between the theoretical and experimental values of fracture toughness, measured within a broad range of temperatures, is no greater than 10–15%. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 39–42, February, 1996.     

Poly(ether ether ketone) with pendent methyl groups as a toughening agent for amine cured DGEBA epoxy resin

A diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was modified with poly(ether ether ketone) with pendent methyl groups (PEEKM). PEEKM was synthesised from methyl hydroquinone and 4,4′-difluorobenzophenone and characterised. Blends of epoxy resin and PEEKM were prepared by melt blending. The blends were transparent in the uncured state and gave single composition dependent T _g. The T _g-composition behaviour of the uncured blends has been studied using Gordon–Taylor, Kelley–Bueche and Fox equations. The scanning electron micrographs of extracted fracture surfaces revealed that reaction induced phase separation occurred in the blends. Cocontinuous morphology was obtained in blends containing 15 phr PEEKM. Two glass transition peaks corresponding to epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum of the blends. The crosslink density of the blends calculated from dynamic mechanical analysis was less than that of unmodified epoxy resin. The tensile strength, flexural strength and modulus were comparable to that of the unmodified epoxy resin. It was found from fracture toughness measurements that PEEKM is an effective toughener for DDS cured epoxy resin. Fifteen phr PEEKM having cocontinuous morphology exhibited maximum increase in fracture toughness. The increase in fracture toughness was due to crack path deflection, crack pinning, crack bridging by dispersed PEEKM and local plastic deformation of the matrix. The exceptional increase in fracture toughness of 15 phr blend was attributed to the cocontinuous morphology of the blend. Finally it was observed that the thermal stability of epoxy resin was not affected by the addition of PEEKM.     

Improvement of mechanical properties of Poly (<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid) by blending of lysine triisocyanate

LTI was attempted to modify the microstructure of poly (l-lactic acid) (PLLA) and to improve its mechanical properties in this study. Bending modulus, strength and fracture toughness of PLLA/LTI were evaluated, and compared to those of the base PLLA to assess the effectiveness of LTI blending. Effect of LTI addition on fracture micromechanism was also investigated by observing and comparing the fracture surfaces of PLLA/LTI and PLLA using a field emission scanning electron microscope (FE-SEM). Experimental results showed that the bending properties such as the bending modulus and the strength are effectively improved due to polymerization of PLLA molecules by LTI blending. The fracture toughness value was also improved due to increase of ductile deformation, i.e., energy dissipation in the crack-tip region.     

Preparation and properties of pressureless-sintered porous Si3N4

Wave-transparent materials used at high temperature environment generated by high supersonic and hypersonic speeds must possess excellent mechanical property. In this paper, porous Si₃N₄ ceramics with high strength were fabricated by low molding pressure (10 MPa) and pressureless sintering process, without any other pore forming agents. The sintering behavior and the effect of porosity on the mechanical strength and dielectric properties were investigated. The flexural strength of porous Si₃N₄ ceramics was up to 57–176 MPa with porosity of 45–60%, dielectric constant of 2.35–3.39, and dielectric loss of 1.6–3.5 × 10⁻³ in the frequency range of 8–18 GHz, at room temperature. With the increase of porosity, the flexural strength, dielectric constant, and dielectric loss all decreased.     

Effects of organic additives on microstructure and mechanical properties of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Green bodies of porous Si₃N₄ ceramics were shaped by extrusion technique using different organic additives as binder during extrusion molding. Different porosity, microstructures and mechanical properties after the extrusion, drying, debinding and sintering stages were investigated. The solid slurry content of 70–75% and extrusion pressure of 0.5–1.0 MPa had played a decisive role in the smooth realization of extrusion molding. The porous Si₃N₄ ceramics were obtained with excellent properties using 4% hydroxypropyl methyl cellulose (HPMC) as binder and polyethylene glycol (PEG) of molecular weight, 1000, as plasticizer with a density of 1.91 g cm⁻³, porosity of 41.70%, three-point bending strength of 166.53 ± 20 MPa, fracture toughness of 2.45 ± 0.2 MPa m^1/2 and Weibull modulus (m) of 20.75.     

Epoxy composites with 200 nm thick alumina platelets as reinforcements

Composites were prepared by dispersing Alumina platelets of polygonal shape having a thickness of 200 nm and size of 5–10 μm in epoxy (LY 556) matrix using sonication. Good dispersion of the platelets was observed through scanning electron microscopy (SEM). The quasi-static plane-strain fracture toughness and tensile properties of the composites were determined for platelet volume fraction varying from 0% to 10%. The results indicated that addition of the platelets give considerable improvement in fracture toughness and good improvement in the elastic modulus of epoxy. For 10% volume fraction of the platelets, the fracture toughness improved by 110% where as the improvement in elastic modulus was 78%. However there was an associated reduction of 53% in tensile strength and 73% in failure strain. SEM of fractured surface was carried out to understand the various mechanisms responsible for the improvement in fracture toughness. By appropriately accounting for the orientation and stacking effects of the platelets, the applicability of predictive models, such as the Halpin-Tsai and Mori-Tanaka, for estimating the composite modulus is demonstrated.     

Micro- and macro-mechanical testing of transparent MgAl2O4 spinel

Advanced transparent ceramics with high chemical and thermal stability are gaining increasing interest as replacement of glass-based materials in technical window applications. The mechanical reliability and performance of transparent MgAl₂O₄ with a grain size of 5 μm has been characterized at ambient temperature using micro-mechanical indentation and macroscopic bending tests. The measurements focused on elastic modulus, fracture toughness, crack kinetics, and strength, the latter analyzed with Weibull statistics. The effect of slow crack growth is assessed using a strength–probability–time plot. Complementary fractography by optical, confocal and scanning electron microscopy provided a correlation between failure origin and fracture stress. The results and reliability aspects are discussed in terms of linear elastic fracture mechanics.     

Preparation and properties of unidirectional boron nitride fibre reinforced boron nitride matrix composites via precursor infiltration and pyrolysis route

The unidirectional boron nitride fibre reinforced boron nitride matrix (BN_f/BN) composites were prepared via the precursor infiltration and pyrolysis (PIP) route, and the structure, composition, mechanical and dielectric properties were studied. The composites have a high content and fine crystallinity of BN. The density is 1.60 g cm⁻³ with a low open porosity of 4.66%. The composites display good mechanical properties with the average flexural strength, elastic modulus and fracture toughness being 53.8 MPa, 20.8 GPa and 6.88 MPa m^1/2, respectively. Lots of long fibres pull-out from the fracture surface, suggesting a good fibre/matrix interface. As temperature increases, both of the flexural strength and elastic modulus exhibit a decreasing trend, with the lowest values being 36.2 MPa and 8.6 GPa at 1000 °C, respectively. The desirable residual ratios of the flexural strength and elastic modulus at 1000 °C are 67.3% and 41.3%, respectively. The composites have excellent dielectric properties, with the average dielectric constant and loss tangent being 3.07 and 0.0044 at 2-18 GHz, respectively.     

Improvement in the mechanical properties of polycrystalline beta-alumina via the use of zirconia particles containing stabilizing oxide additions

Polycrystalline beta-alumina ceramics containing yttria-doped zirconia particles have been produced by hot-pressing and “two-peak” sintering schedules. With the former fabrication process, both a chemical reaction involving sodium metazirconate and α-alumina, and a direct mixing route were employed. The mechanical properties of the ceramics produced by the direct mixing route were superior to those produced by the chemical route. The maximum amount of tetragonal zirconia retention, and thus fracture toughness, obtained using direct mixing occurred for additions of 4wt% yttria-doped zirconia. An increase of ∼ 124% in the fracture toughness was obtained compared with the pure beta-alumina ceramic. Transfer of this fabrication route to a pressureless sintering schedule was less successful owing to difficulties in attaining full densification. The increases in strength observed with introduction of second phase zirconia could be attributed to an improvement in the degree of densification achieved, and the maximum increase in toughness was only ∼27%.     

Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO₂ mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H₂O/Al₂O₃ molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.     

Characterization and analysis of CaO–SiO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript> ternary system ceramics

The dielectric, thermal and mechanical properties of CaO–SiO₂–B₂O₃ ternary system ceramics by solid-phase method have been carried out and quantitive analysis been examined by X-ray diffraction (XRD) patterns. The results showed that the major crystalline phase of CaO–SiO₂–B₂O₃ ternary system ceramics was wollastonite (about 90 wt%) which existed at the temperature ranging from 950 to 1,100 °C. It is also observed that wollastonite could be transformed to pseudowollastonite at 1,200 °C. In addition, with increase in calcination temperature, the amount of wollastonite increases. When the sintering temperature is at 1,100 °C, the amount of wollastonite has a maximum value of 92.7 wt%. Accordingly, CaO–SiO₂–B₂O₃ ternary system ceramics achieved excellent properties at 1,100 °C, such as dielectric constant of 8.38, dielectric loss of 1.51 × 10⁻³ at 1 MHz, linear thermal-expansion coefficient (300 K) of 6.68 × 10⁻⁶/K, bending strength of 121.75 Mpa. Analysis of the mechanical and dielectric properties showed that the measured bending strength, dielectric constant and loss of CaO–SiO₂–B₂O₃ ternary system ceramics can be substantially modified and improved by controlling the sintering temperature, in particular due to the amount of wollastonite crystalline phase and size of grains.     

Polypropylene/SiO<Subscript>2</Subscript> nanocomposites filled with different nanosilicas: thermal and mechanical properties,morphology and interphase characterization

Untreated and surface-treated SiO₂ nanoparticles with different alkyl chain length (described as C0, 3C1, C8 and C16 according to the number of carbon atoms) on particle surface were used as fillers for isotactic polypropylene (iPP). The iPP/SiO₂ composites containing 2.3 vol% of nanoparticles were prepared by melt blending and injection moulding. The dispersion quality of nanoparticles in matrix was examined using scanning electron microscopy (SEM). The crystallization behaviour of iPP was examined using differential scanning calorimetry (DSC). The mechanical properties of all samples were characterized by tensile test, compact tension (CT) test and dynamic mechanical thermal analysis (DMTA). The particle–matrix interphase behaviour was also examined and discussed. SEM images show that different silicas show different dispersion quality in matrix due to different hydrophobicity. The crystallinity and spherulite size of matrix are overall decreased in composites. The tensile properties of iPP/SiO₂ composites show clear relationship with alkyl chain length on particle surface, i.e. increasing alkyl chain length leads to decreased tensile modulus but increased tensile yield strength and strain, indicating increased interfacial interactions with increased alkyl chain length. The 3C1-composite shows the highest fracture toughness with an improvement by 9% compared to neat iPP, whereas the other composites show decreased values of fracture toughness.     

Morphology and mechanical properties of polyacrylonitrile/attapulgite nanocomposite

The major objective of this work is to understand the effects of attapulgite (AT) on the mechanical properties of polyacrylonitrile (PAN)/AT nanocomposite film. The well dispersed but irregularly distributed AT nanoparticles in the matrix was observed by scanning electron microscopy (SEM) and UV–vis spectra. The mechanical properties were investigated by means of tensile tests and dynamic mechanical analyses (DMA). The results showed that the incorporation of AT significantly improved the tensile strength and modulus of the PAN matrix. The fracture morphologies analysis has further suggested that small amount of AT nanorods may slide and orient along the tensile direction, resulting in homogenous stress transfer, thus increase the toughness of the PAN. However, the nanorods network formed in high AT content sample probably hindered the deformation of the matrix and generated the stress concentration points, leading to the remarkable increase of embrittlement of the samples. Following this concept, the volume of the constrained polymer chains was also calculated with DMA data and showed the good correlation with conclusion drawn in the tensile tests.     

A hierarchical study of the mechanical properties of gypsum

The flexural strength of gypsum is reported for freestanding single crystals in three-point bending carried with a nanoindenter. The elastic modulus, splitting tensile strength, and fracture toughness of monolithic gypsum consisting of interlocking needle-like microcrystals are also reported as functions of porosity and accelerator addition. This study shows that geometric configurations, in addition to porosity, affect the mechanical properties of gypsum. The properties are improved by 50–100% when the crystal network changes from needle aggregates to one made up of homogeneous randomly oriented single crystals. An Ashby geometric model for open-cell foams is adopted to link the properties of the individual crystals and the bulk properties. The lower and upper bounds of the measured elastic modulus are in accordance with bending-dominated behavior and stretch-dominated behavior predicted by the model, respectively. However, the strength of gypsum is much lower than values predicted by the model, which is based failure on fracture of individual crystals, suggesting that the strength of monolithic gypsum may be instead controlled by the failure of weak intercrystalline contacts.     

Three-dimensional fractal analysis of fracture surfaces in titanium–iron particulate reinforced hydroxyapatite composites: relationship between fracture toughness and fractal dimension

Fractal dimension has been considered as a measure of fracture surface roughness of materials. Three-dimensional (3D) surface analysis is anticipated to provide a better evaluation of fracture surface toughness and fractal dimension. The objective of this study was to quantify the fracture surfaces and identify a potential relationship between fracture toughness and fractal dimension in a new type of core–shell titanium–iron particulate reinforced hydroxyapatite matrix composites using SEM stereoscopy coupled with a 3D surface analysis. The obtained results showed that both fracture surface roughness and fractal dimension increased with increasing amount of core–shell Ti–Fe reinforcing particles. The fractal dimension was observed to be a direct measure of fracture surface roughness. The fracture toughness of the composites increased linearly with the square root of fractal dimensional increment (i.e., followed the Mecholsky–Mackin equation well) due to the presence of Ti–Fe particles along with the effect of porosity in brittle materials. The 3D fractal analysis was suggested to be a proper tool for quantifying the fracture surfaces and linking the microstructural parameter to fracture toughness.